The invention relates to a blade construction with a vortex generator in the form of an at least partial surface property.
These types of blade constructions are found for example in rotors and blade arrays (rotor blades and/or guide blades) of low-pressure turbines of an aircraft propulsion device.
Influencing a flow, especially the boundary layer close to the surface, through a vortex generator (also called turbulators) is a much investigated topic at least on the level of research. Basically, the boundary layer originates from the wall friction of the flowing particles and forms the flow-related bridge between the profile and the ideal flow that is not affected by the wall friction at some distance from the profile wall being flowed around. The thickness of the boundary layer in this case is a function of the Reynolds number. This thickness of the boundary layer increases continuously with an increasing path length of the flow along the profile wall. Eventually the flow particles begin to abandon the laminar flow behavior (laminar boundary layer) and execute more or less strong lateral movements (turbulent boundary layer). The transition from the laminar boundary layer to the turbulent boundary layer (also called transition range) depends in this case on a series of influencing variables, among them the surface roughness of the profile wall being flowed around, pressure gradients, disturbances to speed and pressure of the outer flow as well as the local Reynolds number.
With a similar speed progression along the outer flow, a turbulent boundary layer generates more frictional resistance than a laminar boundary layer, but, in contrast, has a lower separation tendency. The frictional resistance and pressure distribution around the profile that is changed by the separation cause the profile loss. Such a separation of the boundary layer particularly on a profile upper side or inlet side essentially occurs when particles on flow paths in the boundary layer close to the profile wall cannot be decelerated any further because of flow energy that is too low. They yield laterally as a result and then a so-called separation bubble forms, as shown schematically in FIG. 1. With a dropping Reynolds number, the separation grows more and more in terms of its length until it reaches the area downstream from the rear edge of the profile so that the diversion required by the profile can no longer be achieved. In the lateral center downstream from the separation near the profile wall, no flow particle ever reaches the profile surface again; the flow can no longer follow the profile and breaks off so to speak.
In order to positively influence the break-off behavior of the flow along a profile and the profile loss by controlling the size of the separation bubble, various approaches for arranging fixed turbulators have been pursued in the prior art, all of which have the objective of allowing the laminar boundary layer to transition to a turbulent boundary layer further upstream on the profile. Two examples of these types of turbulators are depicted in the attached FIG. 2. Consequently, the possibility exists of generating turbulators by means of sharp-edged projections on the upper side of the profile or by means of sharp-edged notches (recesses) in the wall being flowed around.
However, in the case of guide blades and rotor blades of a turbo device in particular of a low-pressure turbine, sharp-edged turbulators configured in this way have proven to be disadvantageous insofar as they reduce the separation bubble in an advantageous way only at greater flying altitudes, for instance the cruising altitude an aircraft, because of the low flow Reynolds numbers there and high blade stress, by promptly making the flow turbulent and thereby improving the profile loss and the efficiency, but increase the losses near the ground. In addition, in the case of sharp-edged turbulators, manufacturing, coating and service life are extremely critical. As a result, currently there is no known practical application of turbulators in an engine.
In this regard, the inventor's European Patent Document No. EP 132 638 B1 is itself cited as relevant prior art with respect to the present invention. An axially traversed blade array of a turbine is known, whose blade profile is configured such that the flow is accelerated along the majority of the inlet-side profile surface up to a speed maximum in the region of the channel narrow surface and decelerated downstream from this up to the rear edge of the profile. Every blade is provided with an interference edge, which is arranged downstream from the speed maximum on the inlet side of the blade in the region of the decelerated flow and extends essentially over the entire blade height (distance from the blade root to the blade tip).
In order to reduce the negative effects cited at the outset of a sharp-edged turbulator on the main flow (increase in the frictional loss) particularly in the case of high Reynolds numbers, EP 132 638 B1 provides among other things for the interference edge to be profiled in a saw-toothed manner in a plane tangential to the profile surface. A reduction in the required edge height is supposed to be achieved by this in order to thereby reduce the frictional losses in the case of higher Reynolds numbers. Despite these positive effects, the problems of complicated manufacturing and lower service life remain unsolved.
In view of this prior art, the object of the present invention is creating a blade of a turbo device with a generic vortex generator (turbulator) which can be manufactured simply and has a longer service life.
This object is attained by a blade of a turbo device with an undulating (edgeless) vortex generator extending along the blade.
As a result, the invention is a blade construction of a turbo device, preferably of a blade array arrangement of a low-pressure turbine, on whose inlet-side profile surface downstream from the speed maximum, if applicable, also beginning in the region of the speed maximum, a vortex generator is arranged, preferably in the form a partial geometric surface property. The vortex generator is formed further preferably by a surface undulation with at least one wave, whose wave tail runs in the form of a wave trough and a wave peak approximately in the blade vertical direction.
Expressed more concretely, sharp-edged fixtures and modifications to the blade inlet side are dispensed with in order to promptly make the flow (boundary layer) turbulent. Instead one or more edgeless waves are arranged or configured on the surface of the inlet side. The advantage of doing this is that the manufacturability, coatability and service life of the blade are improved as compared to the known prior art. The effect of the undulating vortex generator on the flow behavior in the area near the surface is comparable to the known sharp-edged turbulators. With high Reynolds numbers in particular near the ground, however, a lower loss of efficiency from friction is observed. In addition, an acoustic reduction in engine noise of 1-2 dB can be realized, i.e., a 10-20% reduction in pressure amplitude.
An important advantage of the turbulator according to the invention is the direct manufacturability within the manufacturing process of a blade of a turbo engine. In the case of cast gas turbine blades, this surface property/structure may be integrated directly into the casting model without noticeable additional costs. However, subsequently introducing the wave structure according to the invention is also possible. This applies not only to new fabrication, but also to cases of overhauling. As a result, a retrofit is also possible. Forming processes as well as metal cutting processing methods, e.g., such as compression, grinding and/or milling but also electro-chemical removal, can be used to subsequently introduce the wave structure.
The smooth wave shape according to the invention has more advantages as compared with the prior art such as, e.g., U.S. Pat. No. 6,416,289 or European Patent Document No. EP 1 081 332 A1, which describes a strip with increased roughness.
Because the roughness strip according to the prior art is a region with irregular, increased roughness, which is not produced in a normal fabrication process (for example with cast gas turbine blades), this roughness must be applied subsequently. In contrast, the smooth wave turbulators can be introduced directly during the normal fabrication process (the term “smooth” should be understood as a surface quality, which corresponds to the other areas of the turbine blade). An additional increase in roughness as compared with the remaining blade regions is not required for the wave turbulators.
Even the functionality of the wave turbulators according to the invention is basically different from locally increased roughness. If, with increased roughness, at least the peaks project from the layer of the constant shear stress gradient (near the wall), flows in the region of the maximum shear stress (transverse and longitudinal vortices) are generated in the boundary layer, which directly produce three-dimensional flow structures with premature transition (i.e., further upstream than without roughness). In contrast to this, the wave turbulators according to the invention stimulate instabilities in the boundary layer flow, which produce Görtler longitudinal vortices in the concave portion of the wavelength, without blending these with additional vortices. As a result, the disadvantageous effect of the wave turbulators with higher Reynolds numbers, such as those that occur, e.g., in engines near the ground, is less than with vortexing turbulators having a conventional design.
Furthermore, it has been shown that in a preferably long-wave embodiment, a targeted excitation of the boundary layer instability is achieved. For this purpose, the wavelength is put in a ratio of λ/sges=0.05 to 0.25 to the inlet side. The amplitude is put in a ratio of a/sges=0.0002 to 0.0040 to the inlet side length. The number of sinusoidal waves is preferably between 1 and 4 and is in a range of ±0.25 s/sges around the position of the narrow section on the inlet side of the blade. However, the optimum values within the aforementioned range depend in this case on the flow conditions like Reynolds number, Mach number, load, etc. The expression “sges” in this case means the inlet-side arc length, measured from the axially forwardmost forward edge point to the rearmost rear edge point.
Moreover, it is preferably possible that in a short-wave embodiment, a targeted excitation of instabilities is likewise achievable by means of frequencies with moderate excitation rates. For this purpose, the wavelength is put in a ratio of λ/sges=0.005 to 0.05 to the inlet side. The number of sinusoidal waves is then preferably between 2 and 15. The remaining settings correspond to the foregoing description.
In addition, it is advantageous to provide on a turbine blade (Maoutlet=0.65 and Reoutlet=200,000) with presumably high stage load (deflection>100°) and high blade load (high lift, uncertainty value>1.0) 3 sinusoidal waves of wavelength λ/sges=0.08 with constant amplitude a/sges=0.001, whose last wavelength ends at the location of the laminar separation. The wavelengths in this case extend 20%-80% beyond the span width extension.
Alternatively, in this case the wavelength corresponding to a short-wave embodiment can also be λ/sges=0.03 with a turbine blade with Reoutlet=400,000.
Finally, in the case of a compressor blade (Maintake=0.5 and Reintake 0 300,000) with a laminar/turbulent separation bubble on the inlet side, this can be formed with three sinusoidal waves of wavelength λ/xges=0.05 with the same amplitude a/xges=0.0002, wherein its last wavelength ends at the location of the laminar separation. The expression “xges” here signifies the axial array width in the center section. The wavelengths also extend 20%-80% beyond the span width extension.
The invention is described in greater detail in the following on the basis of a preferred exemplary embodiment making reference to the accompanying drawings.